Hillslopes cover the majority of Earth's land surface. Understanding the processes by which hillslopes evolve is important for agriculture, landslide hazard mitigation, nutrient transport through ecosystems, and predictions of how landscapes change through time. This project was focused on a big question in hillslope geomorphology (the study of how hillslopes erode and change over time): How does the geology in which the landscape is being developed influence the evolution of the landscape? We used fieldwork, computer modeling and mathematical theory to explore in particular how "rocky" hillslopes evolve.

We have explored how hillslopes develop in a particular type of rock, one in which resistant layers of rock are sandwiched between layers of soft, more erodible rock.  We focused on hogbacks, which are developed in tectonically tilted sedimentary rocks that occur in many sites around the world.  Hogbacks are so-called because they resemble the ridge or spine of a hog?s back.  We conducted fieldwork at Shiprock, New Mexico, and in Boulder, Colorado, and found that hogback hillslopes are usually covered in large blocks of rock derived from the resistant layer, and have a shape in which topographic slope is highest at the top and lowest at the bottom. They are bowl-shaped. We measured both the sizes of boulders and the slope, and found that the size of the boulders and the slope are related. The hillslope is steepest where the boulders are biggest. We also observed that soil builds up behind big blocks, and hypothesized that this must play some important role in shaping the hillslope.

We next created a computer model of hogback evolution, inspired by our fieldwork. Our challenge was to reproduce the chief observations we had made in the real world: the bowl-like shape of the topographic ramp below the cliffs, the build-up of soil upslope of the boulders, and the decline in the sizes of the boulders. Our model tracks how different rock types break down to produce soil (weathering), and how both the soil produced by weathering and big blocks move downhill. As previous models have assumed that there is only fine-grained soil on the hillslope, ours is the first model to track these big blocks. Our results (Figure 1) showed that the blocks are necessary to capture the shape of hogbacks observed in the field. In the model, blocks cause soil to dam up behind them, steepening the local slope. Blocks in the model also weather and move downslope over time, and therefore get smaller with distance downhill. This leads to a decrease in slope downhill, which matches the observed bowl-shape of these hillslopes. This is the first time the shape of these features has been explained. We also developed theory that predicts how hillslopes will evolve in rocks that have different tilts (from vertical, to tilted, to horizontal), which will guide future fieldwork and research.

These model results inspired collaboration with scientists who were studying how big blocks of rock influence rivers.  As big blocks of rock found in a river ultimately come from the adjacent hillslopes, this inspired development of a computer model of river canyon evolution that honored these various roles of big blocks. River canyons are common across the world, and are important because they control how landscapes erode in response to both tectonics and climate. We showed that big blocks that fall into the river from the hillslope can stall erosion of the river, which in turn stalls erosion of the hillslopes. The hillslope and river parts of a landscape are therefore intimately linked. This portion of the project has implications for the history of other landscapes. For example, our results are important to those who study the Grand Canyon and how and when it was formed.

 While we have long attempted to "read" the geology of a landscape from images taken from airplanes and satellites, we have lacked the detail in our models of landscape evolution that could address the geological reality of a site. This project has made important strides toward understanding how Earth?s surface changes over time. We are moving from the simple classification of topographic forms to the questions of "how" and "why" they emerge in a specific geological setting.  We have begun to ask just what about the geology, from the rocks themselves to their orientation, matters in making a landscape. This greatly improves our ability to predict how the surface will change in the future, especially in the presence of large blocks of rock that can be hazardous to humans.

This project has also involved community outreach, including talks for the local general public, hands-on demonstrations for children, and field trips for both diverse undergraduate students and the general public. Finally, the computer models produced in this project are freely available online, and may be used as educational materials for students.

Last Modified: 11/06/2018
Modified by: Robert S Anderson